This invention relates generally to fuel dispensing pumps, and more particularly to a control system for such pumps, especially to such a control system which utilizes a personal computer to exchange information with a smart fuel pump, including a dispenser and/or a user debit or credit card accepter device. More particularly, the invention relates to an interface unit to regulate the exchange of information between the personal computer and the smart fuel pump.
Smart fuel pumps have been developed which are capable of receiving commands from a remote control device, such as a computer. Such commands may include a limit on the amount of gas that may be dispensed, the limit relating to the prepaying of that amount by the customer, a price per unit volume of gasoline, and the like. Such fuel pump may additionally provide status information to the computer such as the amount of fuel dispensed, whether the pump is active or "down", or the like. In addition, to having a dispenser, such smart fuel pump may additionally include a card accepter which reads a magnetic strip on a user debit card, or credit card, to allow the user to prepay for the gasoline purchase from the location of the fuel pump, remote from the computer/cash register.
An interface unit is required to handle the communication from the computer/cash register to the fuel pumps. Because of the physical separation, a universal asynchronous receiver/transmitter (UART) is typically provided to transmit and receive information to/from the remote fuel pumps which are arranged in a current-loop with each fuel pump assigned a unique address. Data exchange between the computer and adaptor box is typically in serial fashion using a standard interface format, such as an RS-232 format. This arrangement has proved to be extremely slow because all data must be converted to serial format and communicated one word at a time. Furthermore, the multiple fuel pumps on the current loop may only be addressed one device at a time from the computer to the interface unit which, in turn processes the information prior to sending it to the fuel pumps, and visa versa. This arrangement is not only exceptionally slow, but is also unreliable. A fault in any one dispenser or card reader can shut down the entire current loop, which typically is the entire system. Furthermore, it is inflexible in that any modifications to the system must be installed by a field-service representative who must physically gain access to the adapter box.
One form of computer that is gaining universal acceptance is the personal computer, or PC. A PC is especially adapted to controlling a fuel dispensing terminal because the PC may additionally handle the cash register function, payroll and inventory for the associated convenience store. A PC typically includes an expansion bus and a plurality of edge card connecters for allowing peripheral devices to directly interface with the PC. Interface circuits engaging the PC bus typically utilize a dual-ported direct memory access (DMA) in which a single memory device is accessible from the PC through the first port or a microprocessor, residing on the interface board through the second port. The direct memory access (DMA) occupies a location in the memory map of the PC and is under the control of the PC, with access granted to the on-board microprocessor by the PC through interrupts generated by the on-board microprocessor. This arrangement has many drawbacks. Not only does the DMA occupy space in the PC memory map, but the supervision of the DMA by the PC is a considerable task, which slows multi-tasking functions within the PC. Furthermore, the on-board microprocessor is required to operate at the same clock speed as the PC, whether or not this is most efficient for the functions being performed by the microprocessor. On a practical level, the DMA chip-set is expensive. Furthermore, there is only a defacto standard for PC bus format. Because there are no true industry standards, there are risks that the caveats required for the DMA may not be completely satisfied by all PCs. Therefore, it cannot be ensured that a DMA-based interface unit is truly universally compatible with all PC's.